U.S. patent number 5,596,199 [Application Number 08/383,959] was granted by the patent office on 1997-01-21 for passive solid state microdosimeter with electronic readout.
This patent grant is currently assigned to Clemson University. Invention is credited to W. Joseph Beauvais, Peter J. McNulty, Wanda K. Moran, Robert A. Reed, David R. Roth.
United States Patent |
5,596,199 |
McNulty , et al. |
January 21, 1997 |
Passive solid state microdosimeter with electronic readout
Abstract
Apparatus and method for qualitatively and quantitatively
analyzing a complex radiation field are provided. A passive
microdosimetry detector device records the energy deposition of
incident radiation using an array of microstructure non-volatile
memory devices. Each microstructure non-volatile memory device is
capable of storing a predetermined initial charge without requiring
a power source. A radiation particle incident to a microstructure
non-volatile memory device is termed an "event". Each such event
may generate a charge within a sensitive volume defined by the
microstructure non-volatile memory device. The charge generated
within the sensitive volume alters the stored initial charge by an
amount falling within a range corresponding to the energy deposited
by certain particle types. Data corresponding to such charge
alterations for a plurality of microstructure non-volatile memory
devices within an array of such devices are presented to a
qualitative analyzing device. The qualitative analyzing device
converts the data to a spectral analysis of the incident radiation
field by applying ICRP-recommended weighting factors to individual
events or approximations thereof.
Inventors: |
McNulty; Peter J. (Sseneca,
SC), Beauvais; W. Joseph (Central, SC), Roth; David
R. (Pendleton, SC), Moran; Wanda K. (Phoenix, AZ),
Reed; Robert A. (Clemson, SC) |
Assignee: |
Clemson University (Clemson,
SC)
|
Family
ID: |
23515474 |
Appl.
No.: |
08/383,959 |
Filed: |
February 6, 1995 |
Current U.S.
Class: |
250/370.07;
250/370.06 |
Current CPC
Class: |
G01T
1/026 (20130101); G01T 1/245 (20130101) |
Current International
Class: |
G01T
1/00 (20060101); G01T 1/02 (20060101); G01T
1/24 (20060101); G01T 001/24 (); G01T 001/02 () |
Field of
Search: |
;250/370.06,370.07 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
1990 Recommendations of the International Commission on
Radiological Projection pp. 79-89, Nov. 1990, New York, New York.
.
D. R. Roth et al., Solid State Microdosimeter for Spacecraft
Applications, 1993 Clemson, South Carolina. .
P. J. McNulty, Radiation Exposure, Dosimetry and Risks, pp.
514-519, Department of Physics and Astronomy, Clemson, South
Carolina. .
Inside AMD's CMOS EPROM Technology, pp. 2-3 through 2-16, Jul.
1993. .
C. A. Sondhaus et al., Cell-Oriented Alternatives to Dose, Quality
Factor, and Dose Equivalent for Low-Level Radiation, pp. 35-48,
Heath Physics, Jul., 1990. .
M. N. Varma et al., Empirical Evaluation of Cell Critical Volume
Dose vs. Cell Response Function for Pink Mutations in Tradescantia,
pp. 440-450, Upton, NY. .
P. J. McNulty et al, Uncertainties in Radiation Effect Predictions
for the Natural Radiation Environments of Space, Dept. of Physics
and Astronomy, Clemson, South Carolina, USA and NASA, Greenbelt, MD
USA, 1994. .
V. P. Bond et al., A Stochastic, Weighted Hit Size Theory of
Cellular Radiobiological Action, pp. 424-437, Published by the
Commission of the European Committee, Proceedings of 8th Symposium,
Sep. 27-Oct. 1, 1982, Report Eur 8395 EN. .
N. G. Blamires et al., pMOS Dosimeters: Long-Term Annealing and
Neutron Response, pp. 1310-1315, IEEE Transactions on Nuclear
Science, vol. NS-33, No. 6, Dec. 1986. .
P. J. McNulty et al., Comparison of the Charge Collecting
Properties of Junctions and the SEU Response of Microelectronic
Circuits, pp. 1-9, Clemson, South Carolina, Sep. 6, 1990. .
P. J. McNulty et al., Characterizing Complex Radiation Environments
Using More (Monitor of Radiation Effects), Oct. 15-18, 1990. .
P. J. McNulty, Predicting Single Event Phenomena in Natural Space
Environments, pp. 3-1 through 3-83, Clemson University, Clemson,
SC, July 16, 1990. .
R. G. Benson, Small is Beautiful: SAIC's New Dosimeter, Nuclear
Engineering International, p. 18, May 1991. .
R. Fletcher, Electronic Personal Dosimeter Heralds Revolution in
Legal Dosimetry, pp. 19-22, Nuclear Engineering International, May
1991. .
Siemans Advertisement for Electronic Personal Dosimeter, p. 7,
Nuclear Engineering International, May 1991..
|
Primary Examiner: Fields; Carolyn E.
Attorney, Agent or Firm: Dority & Manning, P.A.
Claims
What is claimed is:
1. A passive microdosimetry detector device for recording the
energy deposition of radiation incident thereto and enabling
qualitative analysis of a complex radiation field, comprising an
array of microstructure non-volatile memory devices, a plurality of
said microstructure non-volatile memory devices each defining a
corresponding microstructure sensitive volume within which charge
is generated responsive to incident radiation and wherein at least
one of said non-volatile memory devices is configured to store a
predetermined initial charge such that said generated charge
measurably alters said predetermined initial charge stored by said
at least one microstructure non-volatile memory device.
2. The passive microdosimetry detector as in claim 1, wherein said
plurality of memory devices are each configured to store a
predetermined initial charge such that a charge generated in its
corresponding sensitive volume measurably alters the predetermined
initial charge stored thereby.
3. The passive microdosimetry detector as in claim 1, wherein said
array of microstructure non-volatile memory devices comprises a
semiconductor device.
4. The passive microdosimetry detector as in claim 3, wherein said
semiconductor device is configured to individually address said at
least one microstructure non-volatile memory device.
5. The passive microdosimetry detector as in claim 4, wherein said
semiconductor device comprises an erasable programmable read only
memory chip.
6. The passive microdosimetry detector as in claim 3, wherein said
semiconductor device is configured to permit the individual
measurement of said at least one microstructure non-volatile memory
device by a measurement device.
7. The passive microdosimetry detector as in claim 1, further
comprising shielding about said at least one microstructure
non-volatile memory device so as to simulate specific biological
tissue.
8. The passive microdosimetry detector as in claim 1, wherein each
of said microstructure sensitive volumes is generally not more than
one cubic micrometer.
9. The passive microdosimetry detector as in claim 8, wherein each
of said microstructure sensitive volumes is approximately 0.1
micrometers by 1.0 micrometer by 100 angstroms thick with respect
to the incidence of said radiation.
10. The passive microdosimetry detector as in claim 8, wherein each
of said microstructure sensitive volumes is approximately the size
of a biological cell nucleus.
11. The passive microdosimetry detector as in claim 8, wherein each
of said microstructure sensitive volumes is approximately the size
of a DNA genome.
12. The passive microdosimetry detector as in claim 1, wherein said
predetermined initial charge is stored on an electrically insulated
gate within each said at least one microstructure non-volatile
memory device.
13. The passive microdosimetry detector as in claim 1, wherein said
predetermined initial charge on said at least one microstructure
non-volatile memory device is set relative to a known threshold
charge such that traversal of said known threshold charge from said
predetermined initial charge causes said at least one
microstructure non-volatile memory device to change state.
14. The passive microdosimetry detector as in claim 13, wherein
said at least one microstructure non-volatile memory device is
configure such that incident radiation causes a charge alteration
from said predetermined initial charge towards said threshold
level.
15. A passive microdosimetry system for qualitatively analyzing
radiation incident thereon in a complex radiation field,
comprising:
at least one detector array of microstructure non-volatile memory
devices, a plurality of said devices each defining a corresponding
microstructure sensitive volume within which charge is generated
responsive to incident radiation and wherein at least one of said
microstructure non-volatile memory devices is configured to store a
predetermined initial charge such that said generated charge
measurably alters said predetermined initial charge stored by said
at least one microstructure non-volatile memory device;
a measurement device operatively associated with said detector
array, comprising
communicating device for communicating with said at least one
detector array,
measuring mechanism for measuring said charge generated on said at
least one microstructure non-volatile memory device; and
a qualitative analyzing device operatively associated with said
measurement device for converting said generated charge to a
qualitative analysis of the complex radiation field.
16. The passive microdosimetry system as in claim 15, wherein said
at least one detector array of microstructure non-volatile memory
devices comprises a semiconductor device configured to individually
address said at least one microstructure non-volatile memory
device.
17. The passive microdosimetry system as in claim 16, wherein said
semiconductor device comprises an erasable programmable read only
memory chip.
18. The passive microdosimetry system as in claim 16, wherein said
semiconductor device is configured to permit the individual
measurement of said at least one microstructure non-volatile memory
device by said measurement device.
19. The passive microdosimetry system as in claim 15, wherein said
measuring mechanism is configured to measure said generated charge
without destroying the post-irradiation charge stored on said at
least one microstructure non-volatile devices.
20. The passive microdosimetry system as in claim 15, wherein each
of said microstructure sensitive volumes is generally not more than
one cubic micrometer.
21. The passive microdosimetry system as in claim 20, wherein each
of said microstructure sensitive volumes is approximately 0.1
micrometers by 1.0 micrometer by 100 angstroms thick with respect
to the incidence of said radiation.
22. The passive microdosimetry system as in claim 15, wherein said
measurement device further comprises a charging device for charging
said at least one microstructure non-volatile memory device to said
predetermined initial charge.
23. The passive microdosimetry system as in claim 22, wherein said
predetermined initial charge is stored on an electrically insulated
gate within each said at least one microstructure non-volatile
memory device.
24. The passive microdosimetry system as in claim 22, wherein said
predetermined initial charge on said at least one microstructure
non-volatile memory device is set relative to a known threshold
charge such that traversal of said known threshold charge from said
predetermined initial charge causes said at least one
microstructure non-volatile memory device to change state.
25. The passive microdosimetry system as in claim 24, wherein said
at least one microstructure non-volatile memory device is
configured such that said generated charge alters said
predetermined initial charge towards said threshold charge.
26. The passive microdosimetry system as in claim 25, wherein said
at least one microstructure non-volatile memory device is
configured such that the difference between said altered charge and
said predetermined initial charge corresponds to the energy
deposited by incident radiation on said at least one microstructure
non-volatile memory device during exposure of said at least one
detector array to incident radiation.
27. The passive microdosimetry system as in claim 24, wherein said
measuring mechanism is configured to measure the charge alteration
due to said charge generated by incident radiation by changing the
post-irradiation charge stored on said at least one microstructure
non-volatile memory device at a known rate toward said threshold
charge and determining the time required for said at least one
microstructure non-volatile memory device to change state.
28. The passive microdosimetry system as in claim 27, wherein said
measuring mechanism is configured to change said post-irradiation
charge on said at least one microstructure non-volatile memory
device by optical application of ultraviolet light to said at least
one detector array.
29. The passive microdosimetry system as in claim 27, wherein said
measuring mechanism is configured to change said post-irradiation
charge on said at least one microstructure non-volatile memory
device by the application of electrically generated tunnelling
current to said at least one microstructure non-volatile memory
device.
30. The passive microdosimetry system as in claim 27, wherein said
measuring mechanism is configured to change said post-irradiation
charge on said at least one microstructure non-volatile memory
device by the imposition of an electric field to said at least one
microstructure non-volatile memory device in opposition to a stable
state electric field maintained by said at least one microstructure
non-volatile memory device.
31. The passive microdosimetry system as in claim 15, wherein said
qualitative analyzing device is configured to convert said
generated charge into an estimate of the number of events occurring
within discrete energy bands within the measured area of said at
least one detector array of microstructure non-volatile memory
devices.
32. The passive microdosimetry system as in claim 31, wherein said
qualitative analyzing device is configured to apply sensitive
volume dependent weighting factors to said estimate according to
said discrete energy bands, said weighting factors correlating to
known energy ranges within which known ionizing particles, in
sensitive volumes comparable to said sensitive volumes of said
microstructure non-volatile memory devices, deposit energy so that
a spectral analysis of the complex radiation field is
generated.
33. The passive microdosimetry system as in claim 15, wherein said
measuring mechanism is furthermore configured to measure the
spatial distribution of said generated charge depositions
throughout the measured area of said at least one detector array of
microstructure non-volatile memory devices.
34. The passive microdosimetry system as in claim 33, wherein said
qualitative analyzing device is configured to convert said
generated charge deposition and distribution into an estimate of
the number of events occurring within said measured area of said at
least one detector array of microstructure non-volatile memory
devices within discrete energy bands.
35. The passive microdosimetry system as in claim 34, wherein said
qualitative analyzing device is configured to apply sensitive
volume dependent weighting factors to said estimate according to
said discrete energy bands, said weighting factors correlating to
known energy ranges within which known ionizing particles, in
sensitive volumes comparable to said sensitive volumes of said
microstructure non-volatile memory devices, deposit energy so that
a spectral analysis of the complex radiation field is
generated.
36. The passive microdosimetry system as in claim 15, wherein said
system is configured as a personal sized radiation microdosimetry
device.
37. The passive microdosimetry system as in claim 15, wherein said
system is configured as an area monitor.
38. The passive microdosimetry system as in claim 37, wherein said
system is configured as an area monitor for a space platform.
39. A method for qualitatively analyzing a complex incident
radiation field, comprising the step of:
subjecting to the radiation field a detector array comprised of a
plurality of microstructure non-volatile memory devices having a
corresponding plurality of associated microstructure sensitive
volumes;
individually measuring the charge deposition on at least one of
said microstructure non-volatile devices, said charge deposition
being generated responsive to the incident radiation within the
corresponding microstructure sensitive volume of said at least one
microstructure non-volatile memory device; and
outputting said charge deposition for conversion to a qualitative
analysis of the complex radiation field.
40. The method as in claim 39, further comprising the steps of:
converting said charge deposition into an estimate of the number of
events within discrete energy bands occurring within a measured
area of said array of microstructure non-volatile memory devices;
and
translating said estimate into a qualitative analysis of the
complex radiation field.
41. The method as in claim 40, wherein said qualitative analysis
comprises a spectral analysis.
42. The method as in claim 40, wherein said measuring step further
comprises the step of measuring the spatial charge distribution
throughout said measured area and wherein said converting step
converts said charge deposition and said spatial charge
distribution to said estimate.
43. The method as in claim 40, wherein said translating step
further comprises the step of applying sensitive volume dependent
weighting factors to said estimate according to said discrete
energy bands, said weighting factors correlating to known energy
ranges within which known ionizing particles, in sensitive volumes
comparable to said sensitive volumes of said microstructure
non-volatile memory devices, deposit energy so that a spectral
analysis of the complex radiation field is generated.
44. The method as in claim 42, wherein said translating step
further comprises the step of applying sensitive volume dependent
weighting factors to said estimate according to said discrete
energy bands, said weighting factors correlating to known energy
ranges within which known ionizing particles, in sensitive volumes
comparable to said sensitive volumes of said microstructure
non-volatile memory devices, deposit energy so that a spectral
analysis of the complex radiation field is generated.
45. The method as in claim 39, wherein said measuring step
preserves the stored charge on said at least one microstructure
non-volatile memory device.
46. The method as in claim 45, further comprising the step of
charging, prior to said subjecting step, said at least one
microstructure non-volatile memory device of said array to a
predetermined initial charge set relative to a known threshold
charge such that traversal of said known threshold charge from said
predetermined initial charge causes said at least one
microstructure non-volatile memory device to change state and
wherein said predetermined initial charge may be altered towards
said threshold charge during said subjecting step responsive to
incident radiation; and
wherein said measuring step is further comprised of the steps of
repeatedly reading the state of, and decrementing by a
predetermined value the control gate bias of, each of said at least
one microstructure non-volatile memory devices within a measured
area of said array of microstructure non-volatile memory devices
and determining the control gate bias at which each said
microstructure non-volatile memory device within said measured area
changes state.
47. The method as in claim 46, further comprising the steps of:
converting said control gate bias value at which each said
microstructure non-volatile memory device within said measured area
changes state into an estimate of the number of events within
discrete energy bands occurring within said measured area; and
translating said estimate into a qualitative analysis of the
complex radiation field.
48. The method as in claim 47, wherein said qualitative analysis
comprises a spectral analysis.
49. The method as in claim 39, further comprising the step of
charging, prior to said subjecting step, said at least one
microstructure non-volatile memory device of said array to a
predetermined initial charge, said predetermined initial charge
being altered during said subjecting step responsive to said
incident radiation.
50. The method as in claim 49, wherein said predetermined initial
charge is set relative to a known threshold charge such that
traversal of said known threshold charge from said predetermined
initial charge causes said at least one microstructure non-volatile
memory device to change state.
51. The method as in claim 50, wherein said measuring step further
comprises the step of changing the post-irradiation charge on said
at least one microstructure non-volatile memory device at a know
rate towards said threshold charge and determining the time
required for said at least one microstructure non-volatile memory
device to charge state.
52. The method as in claim 51, wherein said measuring step further
comprises the step of detecting, prior to changing said
post-irradiation charge, any said at least one microstructure
non-volatile memory device having changed state responsive to
incident radiation.
53. The method as in claim 51, wherein said post-irradiation charge
is changed by optically applying ultraviolet light to said detector
array.
54. The method as in claim 51, wherein said post-irradiation charge
is changed by applying electrically generated tunnelling current to
said at least one microstructure non-volatile memory device.
55. The method as in claim 51, wherein said post-irradiation charge
is changed by imposing an electric field to said at least one
microstructure non-volatile memory device in opposition to a stable
state electric field maintained by said at least one microstructure
non-volatile memory device.
56. The method as in claim 39, further comprising the step of
shielding said at least one microstructure non-volatile memory
device with shielding material so that said at least one
microstructure non-volatile memory device simulates specific
biological tissue.
57. A passive microdosimetry system for qualitatively analyzing
radiation incident thereof in a complex radiation field,
comprising:
at least one detector array of microstructure non-volatile memory
device comprising a semiconductor device configured to individually
address at least one of said microstructure non-volatile memory
devices, each of said at least one microstructure non-volatile
memory device being configured to store a predetermined initial
charge thereon and defining a microstructure sensitive volume
within which charge is generated responsive to incident radiation
such that said generated charge individually and measurably alters
said predetermined initial charge stored on said at least one
microstructure non-volatile memory device;
measurement device operatively associated with said at least one
detector array, comprising
communicating device for communicating with said at least one
detector array, and
measuring mechanism for measuring said charge generated on said at
least one microstructure non-volatile memory devices within a
measured area of said at least one detector array; and
a qualitative analyzing device operatively associated with said
measurement device for converting said generated charge into an
estimate of the number of events occurring within discrete energy
bands within said measured area and applying sensitive volume
dependent weighting factors to said estimate according to said
discrete energy bands, said weighting factors correlating to known
energy ranges within which known ionizing particles, in sensitive
volumes comparable to said sensitive volumes of said microstructure
non-volatile memory devices, deposit energy so that a spectral
analysis of the complex radiation field is generated.
58. The passive microdosimetry system as in claim 57, wherein each
microstructure sensitive volume of said at least one microstructure
non-volatile memory device is generally not more than one cubic
micrometer.
59. The passive microdosimetry system as in claim 57, wherein said
predetermined initial charge on said at least one microstructure
non-volatile memory device is set relative to a known threshold
charge such that traversal of said known threshold charge from said
predetermined initial charge causes said at least one
microstructure non-volatile memory device to change state and
wherein said at least one microstructure non-volatile memory device
is configured such that said generated charge alters said
predetermined initial charge towards said threshold charge.
60. The passive microdosimetry system as in claim 59, wherein said
measuring mechanism is configured to measure said charge alteration
due to incident radiation by changing the post-irradiation charge
on said at least one microstructure non-volatile memory device at a
known rate towards said threshold charge and determining the time
required for said at least one microstructure non-volatile memory
device to change state.
61. The passive microdosimetry system as in claim 57, wherein said
system is configured as a personal sized radiation microdosimetry
device.
62. The passive microdosimetry system as in claim 57, wherein said
system is configured as an area monitor.
63. A method for qualitatively analyzing a complex incident
radiation field, comprising the steps of:
charging to a predetermined initial charge at least one
microstructure non-volatile memory device of at least one detector
array of microstructure non-volatile memory devices each having an
associated microstructure sensitive volume, wherein said
predetermined initial charge is set relative to a known threshold
charge so that the traversal of said known threshold charge from
said predetermined initial charge causes said at least one
microstructure non-volatile memory device to change state;
subjecting to the radiation field said at least one detector array;
and
measuring the generated charge on a measured area of said detector
array, said generated charge being responsive to incident radiation
within said microstructure sensitive volumes of the at least one
microstructure non-volatile memory devices within said measured
area, by changing the post-irradiation charge on at least one of
said microstructure non-volatile memory device at a known rate
toward said threshold charge and determining the time required for
each said at least one microstructure memory device to change
state.
64. The method as in claim 63, further comprising the steps of:
converting said generated charge into an estimate of the number of
events occurring within discrete energy bands within said measured
area; and
applying sensitive volume dependent weighting factors to said
estimate according to said discrete energy bands, said weighting
factors correlating to known energy ranges within which known
ionizing particles, in sensitive volumes comparable to said
sensitive volumes of said microstructure non-volatile memory
devices, deposit energy so that a spectral analysis of the complex
radiation field is generated.
65. The method as in claim 63, further comprising the step of
detecting, prior to said changing step, any said at least one
microstructure non-volatile memory device having changed state
responsive to incident radiation.
66. The method as in claim 63, further comprising the step of
measuring the spatial charge distribution throughout said measured
area and wherein said charge deposition and said spatial charge
distribution are converted to said estimate in said converting
step.
Description
BACKGROUND OF THE INVENTION
The present invention relates to solid state microdosimetry and
more particularly to devices and methodology concerning a passive
array of microstructure radiation sensitive volumes which enable
the recording of radiation exposures occurring on a microscopic
level.
The probability that a given exposure to low-level ionizing
radiation will result in significant damage to an organism depends
on the number of ionizations generated within the regions of
biological cells containing DNA, principally the cell nucleus. As a
result, the National Council on Radiation Protection and
Measurement defines, for example, dose equivalent (DE) limits for
work exposure to a specific type of radiation in terms of the
product of the dose, a measure of the number of ionizations per
unit volume expressed in terms of energy deposition per unit mass,
and a quality factor (QF) which depends on the density of
ionizations along the particle's trajectory. See ICRP
"Recommendations of the International Commission on Radiological
Protection," ICRP Publication 60, Annals of the ICRP, 21 No. 1-3,
Pergamon Press, Oxford, 1991. That is:
Dose equivalent is not, however, the only measurement of the
propensity of radiation to damage a given type of irradiated area
or volume. Other measurements have been employed in performing such
a spectral analysis of incident radiation. In particular, as
detailed in the above-referenced ICRP Publication, the ICRP has
defined dose equivalent related concepts of equivalent dose and
effective dose. For ease of explanation, however, dose equivalent
will be hereafter given as the primary example.
Ionization density is normally expressed in terms of the charged
particle's linear energy transfer (LET). Particles with higher LET
values deposit relatively more energy and generate more ionizations
within the biological cell nuclei they traverse. The probability of
a somatic mutation or other biological effect increases with the
LET of the incident radiation until some optimum value and then
falls off at higher LET values. The relationship among dose, dose
equivalent, and cell response is described in more detail in the
scholarly articles by C.A. Sondhaus, V. P. Bond, and L. E.
Feinendegen, "Cell Oriented Alternatives to Dose, Quality Factor,
and Dose Equivalent for Low-Level Radiation," Health Physics 59,
35-48 (1990); V. P. Bond and M. N. Varma, "Low-Level Radiation
Response Explained in Terms of Fluence and Cell Critical Volume
Dose" and "Empirical Evaluation of Cell Critical Volume Dose vs.
Cell Response Function for Pink Mutations in Tradescantia," in
Eight Symposium on Microdosimetry, (Julich, Germany, Commission of
the European Communities, 1983) 423-450.
An instrument capable of measuring the energy deposition in small
microvolumes and assigning quality factors to each event separately
is disclosed in commonly assigned U.S. Pat. No. 5,256,879. The
disclosure of such '879 patent is hereby fully incorporated herein
by reference. The invention disclosed in the '879 patent is an
active microdosimetry device, i.e., one requiring associated
electronics and a constant power source. More particularly, it
records transient events and requires its associated electronics to
store a permanent record of radiation exposure.
While the disclosure of the '879 patent is appropriate and fully
satisfactory in many instances, many other applications for a
microdosimeter, however, favor a totally passive device (i.e., one
requiring no power during exposure). Such is particularly true, for
example, with respect to personnel radiation detection devices and
space applications. Personnel detectors preferably should be small
and light enough to be comfortable and to allow freedom of
movement. The absence of a power source and measurement electronics
as associated with active microdosimeters therefore makes passive
devices relatively more attractive in such situations. Similarly,
the size, weight, and power consumption constraints involved in any
application for use in space make a passive microdosimeter approach
more attractive than an active microdosimeter arrangement.
As a result, a choice between an active microdosimeter and prior
passive dosimeter arrangements requires a choice between accepting
the above-described constraints involved with an active
microdosimeter and the inability of typical previous passive
dosimeters to distinguish among radiation that is likely to cause
damage in biological cells or microelectronic devices.
Therefore, to avoid the necessity of choosing between only such two
devices, it is desirable to have a passive microdosimetry device,
that is, a device capable of calculating dose-equivalent or similar
measure of the propensity of radiation to damage an irradiated area
of interest and having an array large enough to measure exposure
levels as low as a few millirem, yet which requires no individual
detector-associated power source and measurement circuitry. Such a
device would be able, for example, to detect and distinguish
between events generated by neutrons and/or alpha particles. It
would also be beneficial if the device were inexpensive and have
simple on-board instrumentation. Current state of the art
arrangements are generally described below.
There are generally two types of radiation detection instruments:
dosimeters and microdosimeters. Dosimeters measure exposure in
terms of dose. Microdosimeters characterize exposure in terms of
dose equivalent or similar measurement capable of describing the
propensity of incident radiation to damage an irradiated volume.
Both types of instrumentation can be further characterized
according to whether they are active or passive, i.e., according to
whether they require power while recording exposure. A device is
active if power is so required; it is passive if not.
Dosimeters generally characterize radiation exposure in terms of
rads (ergs/gram), which is the dose, or the energy deposited per
unit mass. Dose, in turn, is proportional to the number of
ionizations per unit volume within a given material. As is
explained more fully in U.S. Pat. No. 5,256,879 referenced above,
dosimeters generally do not distinguish events according to the
type of radiation and are limited to measuring exposure in terms of
the amount of energy deposited per unit volume (dose) and the rate
at which that energy is deposited (dose rate).
Dosimeters can be, furthermore, divided into active and passive
devices. The passive devices cumulatively record some effect of the
exposure which when the device is "read" can be translated into
dose. That is, passive devices do not require power to record
events during exposure to the incident radiation field. The
radiation events leave a lasting effect upon the non-powered
devices which a measurement device can later read or which causes
some visible or audible effect upon the device. Passive devices may
include various items such as film badges and thermo-luminescent
dosimeter (TLD) devices.
In contrast, active devices require some type of external power to
detect a radiation event. They may, for example, be used as
integrating devices, such as a pocket dosimeter using an ionization
chamber or a p-n diode, to measure total dose. They may also be
continuously monitored to determine the dose rate as well as the
total integrated dose. In particular, the latter configuration may
be connected to a circuit which provides an audible and/or visible
warning of dangerous levels of dose rate.
One type of active dosimeter employs a RadFET. The RadFET device is
described in some detail in A. G. Holmes-Siedle, L. Adams, N. G.
Blamires, and D. H. J. Totterdel, "PMOS Dosimeters: Long Term
Annealing and Neutron Response", IEEE Transactions on Nuclear
Science NS-33, 1310 (1986). A RadFET dosimeter incorporates a
single transistor which is relatively large in size. As is
generally true of all dosimeters, therefore, the RadFET device
exhibits a large sensitive volume. For a metal oxide semiconductor,
the sensitive volume may be generally defined as that volume about
the junction within which charges (electron/hole pairs) generated
by traversing radiation particles are efficiently collected at the
junction.
As is discussed in more detail in the Detailed Description below,
the likelihood that incident radiation will damage, for example, a
cell nucleus or a DNA genome depends upon the size of the cell or
genome. Thus, to qualitatively analyze an incident radiation field
as to its propensity to cause such damage, the sensitive volume of
the radiation detector should approximate the size of the physical
volume of interest. As a result of its relatively large sensitive
volume, therefore, the RadFet device is incapable of providing a
radiation analysis compatible with ICRP weighting factors.
Although the circuitry used to read the RadFET dosimeter may vary,
a basic method underlying the device is to measure the turn-on
voltage of a PMOS transistor, that is, the voltage which must be
applied between the source and drain of the PMOS transistor to turn
it on. Exposure to radiation changes this turn-on voltage and,
therefore, the RadFET may be used as a dosimeter by monitoring such
change as caused by an incident field.
In particular, ionizing radiation causes a build-up of charge at
the interface between the oxide and the substrate under the gate.
The charge build-up is approximately proportional to the amount of
energy deposited (number of electron-hole pairs generated) within
the oxide under the gate. The gate is maintained at a constant
voltage determined by the circuit and the methodology followed in
reading the dosimeter. The charge deposited as a result of the
radiation, therefore, moves the turn-on voltage either closer to,
or farther away from, the charge held on the gate, depending on the
device configuration. The energy deposition in the device may then
be determined by measuring the difference between gate voltage and
the post-irradiation turn-on voltage and comparing this measure to
the pre-irradiation difference.
Dosimeters using such technology typically use a single transistor
per sensor. Dosimeters based on such design, as is true of
dosimeters generally, fail to distinguish among types of radiation.
As a result, the RadFET technology as currently used is incapable
of monitoring exposure in terms of dose equivalent or similar
measurements.
As is described above, microdosimeters characterize exposure in
terms of dose equivalent or similar measurement capable of
describing the propensity of incident radiation to damage an
irradiated volume. Such devices generally accomplish such a
spectral analysis through the use of microstructure sensitive
volumes that approximate the size of, for example, biological cell
nuclei, DNA genomes, or micro-electronic junctions. One example of
an active microdosimeter is the device disclosed in the
above-referenced U.S. Pat. No. 5,256,879. Another example of an
active device is the gas microdosimeter. Although such latter type
of device employs a relatively large area within which radiation
events are analyzed, microstructure areas are approximated by
varying gas density within the device. Due to size and cost
constraints, gas microdosimeters are not generally practical in,
for example, personnel detection applications.
SUMMARY OF THE INVENTION
The present invention recognizes and addresses various of the
foregoing problems, and others, concerning radiation detection.
Thus, broadly speaking, one principal object of the present
invention is to provide an improved microdosimetry device and
corresponding method for qualitatively analyzing radiation
occurring in a field of complex incident radiation.
A further object of the present invention is to provide a radiation
monitoring device for qualitatively and quantitatively analyzing a
complex radiation field to provide a dose equivalent estimate or
similar measure of the propensity of incident radiation to damage
an irradiated volume.
Yet another object of the present invention is to provide a passive
radiation monitoring device capable of providing a quantitative and
qualitative analysis of an incident radiation field.
It is a more particular present object to provide a radiation
detector and methodology utilizing an array of microstructure
non-volatile memory devices which define radiation sensitive
volumes approximating the size of biological cell nuclei.
Similarly, it is another object to utilize an array of
microstructure non-volatile memory devices which define radiation
sensitive volumes approximating the size of DNA genomes.
Yet another object of the present invention is to provide a
microdosimetry device for spectrally measuring the effects of
radiation upon biological cell nuclei or DNA genomes.
Still another object of the present invention is to provide a
microdosimetry device which utilizes a detector array which may be
partially read, thereby allowing a remaining area to continually
record incident radiation.
A further object is to provide a personal sized microdosimetry
device capable of measuring the dose equivalent, or related
measurement, from an incident radiation field.
Similarly, it is a still further object to provide a personal sized
microdosimetry device which does not require a continuous power
source or associated measurement electronics.
The present invention is equally concerned with improved
methodology corresponding with the above-referenced devices.
Additional objects and advantages of the invention will be set
forth in part in the description which follows, and in part will be
apparent to one of ordinary skill in the art from the description,
or may be learned by practice of the invention. The objects and
advantages of the invention may be realized and obtained by means
of the instrumentalities and combinations particularly pointed out
in the appended claims.
Also, it should be further appreciated that modifications and
variations to the specifically illustrated and discussed features
hereof may be practiced in various embodiments and uses of this
invention without departing from the spirit and scope thereof, by
virtue of present reference thereto. Such variations may include,
but are not limited to, substitution of equivalent means and
features, materials, or steps for those shown or discussed, and the
functional or positional reversal of various parts, features, steps
or the like.
Still further, it is to be understood that different embodiments,
as well as different presently preferred embodiments, of this
invention may include various combinations or configurations of
presently disclosed features, steps, or elements, or their
equivalents, including combinations or configurations thereof not
expressly shown in the figures or stated in the detailed
description.
The present invention utilizes a detector array of microstructure
non-volatile memory devices wherein each microstructure
non-volatile memory device acts as a separate detector. As with
prior art microdosimeters generally, the sensitive volumes of the
individual devices comprise microstructure sensitive volumes that
permit spectral analysis of incident radiation fields utilizing the
ICRP recommended weighting factors to arrive at various
measurements of the likelihood that a given radiation field will
cause damage to a particular sized volume.
Unlike prior art solid state microdosimeters, however, each
sensitive volume of the present invention is generally one cubic
micrometer or smaller, thereby approximating the size of a
biological cell nucleus and approaching the size of a DNA genome.
Although at present the exact dimensions of the respective
sensitive volumes of the different devices which may be practiced
with the present invention are not precisely known, they may be
determined through direct experimentation as discussed generally
below. The above approximation is, however, acceptable for
analyzing radiation with regard to damaging events in the
above-mentioned biological volumes.
Specifically, at such sensitive volume sizes, particles that
deposit similar but distinct energies in larger volumes may deposit
widely varying energies such that the ability and necessity to
distinguish among such particles is eliminated. Consequently, it is
generally only necessary to distinguish among particles making
significantly different energy deposits, as is reflected by the
assignment of weighting factors to ranges of energies by the ICRP
as referenced above. Correspondingly, as is discussed below, the
need to exactly determine the sensitive volume size at these levels
is generally eliminated.
One presently preferred embodiment of the present invention, for
example, is to be employed in personnel radiation measurement
devices near nuclear facilities where the radiation fields of
interest primarily require the distinction between neutrons and
gamma rays, radiation types depositing significantly different
energies within sensitive volumes of such size.
More particularly, the present invention provides an apparatus and
method for passively recording a charge deposition corresponding to
an energy deposition by incident radiation on the individual
devices within the array of microstructure non-volatile memory
devices. Further embodiments of the present invention provide
apparatus and method for measuring such charge deposition,
continuously or occasionally, so that a dose equivalent or similar
estimate can be provided. The apparatus of the present invention
overcomes the drawbacks associated with dosimeters generally
discussed above and overcomes drawbacks associated with active
microdosimeters, such as the need for a continuous power source and
associated measurement electronics.
To achieve the objects and in accordance with the purposes of the
invention, as embodied and broadly described herein, an exemplary
apparatus of the present invention comprises an array of
microstructure non-volatile memory devices, a plurality of such
microstructure non-volatile memory devices each defining a
corresponding microstructure sensitive volume within which charge
is generated responsive to incident radiation and wherein at least
one of the non-volatile memory devices is configured to store a
predetermined initial charge such that the generated charge
measurably alters such predetermined charge stored by the at least
one microstructure non-volatile memory device.
In one preferred embodiment of the present invention, the array
comprises a semiconductor device, and particularly an erasable
programmable read only memory. In another preferred embodiment of
the present invention, the detector array comprises a non-volatile
random access memory.
In another preferred embodiment of the present invention, the
semi-conductor device is configured so that each individual
microstructure non-volatile memory device may be individually
addressed and measured, thereby permitting the measurement of both
the charge deposition in each device and the distribution of
charges throughout the array.
The microstructure sensitive volumes of the microstructure
non-volatile memory devices preferably each define a volume
generally not more than one cubic micrometer. In still another
preferred embodiment of the present invention, such sensitive
volumes each define a volume of approximately 0.1 micrometer by 1.0
micrometer by 100 angstroms thick with respect to the incidence of
the radiation field. It is generally preferred that each of such
microstructure sensitive volumes define a volume approximately the
size of a biological cell nucleus or a DNA genome.
The array of the present invention is configured to be measurably
sensitive to an incident complex radiation field (i.e., a field
subject to various radiation types, such as gamma, alpha, and/or
neutron radiation). In one preferred embodiment of the present
invention, a plurality of microstructure non-volatile memory
devices comprise a measured area of the array, each holding a
predetermined initial charge prior to exposure to the complex
radiation field. In this embodiment, predetermined initial charge
is stored on an electrically insulated gate embedded within each of
microstructure non-volatile memory devices. Such charge is set
relative to a known threshold charge such that traversal of the
known threshold charge from the predetermined initial charge causes
the microstructure non-volatile memory device to change state. In
operation, incident radiation may cause a charge alteration within
each irradiated device from the predetermined initial charge toward
the threshold level.
To further achieve the objects and in accordance with the invention
as embodied and broadly described herein, a passive microdosimetry
system for quantitatively analyzing radiation incident thereon in a
complex radiation field is provided. The system comprises at least
one detector array of microstructure non-volatile memory devices, a
plurality of such devices each defining a corresponding
microstructure sensitive volume within which charge is generated
responsive to incident radiation and wherein at lease one such
microstructure non-volatile memory device is configured to store a
predetermined initial charge such that the generated charge
measurably alters the predetermined initial charge stored on the at
least microstructure non-volatile memory device; a measurement
device operatively associated with the detector array, comprising a
communicating device for communicating with the at least one
detector array, and a measuring mechanism for measuring the
generated charge on such at least one microstructure non-volatile
memory device; and a qualitative analyzing device operatively
associated with the measurement device for converting the generated
charge to a qualitative analysis of the complex radiation
field.
Furthermore, the difference between the post irradiation charge and
the predetermined initial charge corresponds to the charge
deposited (i.e., the generated charge) during the array's exposure
to the incident radiation. In particular, in this preferred
embodiment, the measuring mechanism is configured to measure the
charge alteration due to the incident radiation by changing the
post-irradiation charge on the at least one microstructure
non-volatile memory device at a known rate toward the threshold
charge and determining the time required for each at least one
microstructure non-volatile memory device to change state.
Furthermore, in this configuration, the qualitative analyzing
device is configured to convert the charge deposition into an
estimate of the number of events occurring within discrete energy
bands within the measured area of array of microstructure
non-volatile memory devices. The qualitative analyzing device is
further configured in this embodiment to apply sensitive volume
dependent weighting factors to the estimate according to the
discrete energy bands, such weighting factors correlating to known
energy ranges within which known ionizing particles, in sensitive
volumes comparable to the sensitive volumes of the microstructure
non-volatile memory devices, deposit energy so that a spectral
analysis of the complex radiation field is generated.
In yet another preferred embodiment of this invention, the
measuring mechanism furthermore measures the spatial distribution
of charge depositions throughout the measured area. Similarly, the
qualitative analyzing device is further configured in this
embodiment to convert the charge deposition and charge distribution
into an estimate of the number of events occurring within the
measured area within discrete energy bands.
In yet another preferred embodiment of this invention, the
measuring mechanism is configured to change the post-irradiation
charge on the at least one microstructure non-volatile memory
device by optical application of ultraviolet light to the at least
one detector array. In still another preferred embodiment, the
measuring mechanism is configured to change the post-irradiation
charge on the at least one microstructure non-volatile memory
device by the application of electrically generated tunneling
current to the at least one microstructure non-volatile memory
device. In a still further preferred embodiment, the measuring
mechanism is configured to change the post-irradiation charge on
the at least microstructure non-volatile memory device by the
imposition of an electric field to the at least one microstructure
non-volatile memory device in opposition to a stable state electric
field maintained by the at least one microstructure non-volatile
memory device.
In a preferred embodiment of the present invention, the passive
microdosimetry system is configured as a personal sized radiation
microdosimetry device. In another presently preferred embodiment,
the passive microdosimetry system is configured as an area monitor,
for example as an area monitor for a space platform.
There is furthermore provided a method for qualitatively analyzing
a complex incident radiation field. The method comprises the steps
of subjecting to the radiation field a detector array comprised of
a plurality of microstructure non-volatile memory devices having a
corresponding plurality of associated microstructure sensitive
volumes; individually measuring the charge deposition on at least
one of the microstructure non-volatile memory devices, the charge
deposition being generated responsive to the incident radiation
within the microstructure sensitive volumes; and providing the
charge deposition to a means for converting the charge deposition
to a qualitative analysis of the complex radiation field. Such
presently preferred embodiment may further comprise the steps of
converting the charge deposition into an estimate of the number of
events within discrete energy bands occurring within the measured
area of the array and translating the estimate into a spectral
analysis of the complex radiation field.
In a preferred embodiment, the method further comprises the step of
charging, prior to the subjecting step, the at least one
microstructure non-volatile memory device of the array to a
predetermined initial charge, such charge being altered during the
subjecting step responsive to incident radiation. Furthermore, the
predetermined initial charge is set relative to a known threshold
charge such that traversal of the known threshold charge from the
predetermined charge causes the at least one microstructure
non-volatile device to change state. In this presently preferred
embodiment, therefore, the measuring step further comprises the
steps of changing the post-irradiation charge on the at least one
microstructure non-volatile memory device at a known rate toward
the threshold level and determining the time required for each
microstructure non-volatile memory device to change state. In
another presently preferred embodiment of this invention, the
measuring step further comprises the step of detecting, prior to
changing the post-irradiation charge, any at least one
microstructure non-volatile memory device having changed state
responsive to incident radiation.
In still another preferred embodiment of this invention, the
translating step further comprises the step of applying sensitive
volume dependent weighting factors to the estimate according to
such discrete energy bands, such weighting factors correlating to
known energy ranges within which ionizing particles, in sensitive
volumes comparable to the sensitive volumes of the microstructure
non-volatile memory devices, deposit energy so that a spectral
analysis of the complex radiation field is generated.
The accompanying drawings, which are incorporated in and constitute
a part of the specification, demonstrate embodiments of the
invention and taken together with the description, serve to help
explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
A full and enabling disclosure of the present invention, including
the best mode thereof, to one of ordinary skill in the art is set
forth more particularly in the remainder of the specification,
including reference to the accompanying figures, in which:
FIG. 1 illustrates a perspective view of an exemplary embodiment of
the present invention used particularly as a personal radiation
microdosimetry device;
FIG. 2 is a perspective view of one preferred embodiment of the
detector portion of the present invention;
FIG. 3 is a cross-sectional view of an exemplary microstructure
non-volatile memory device comprising one element in a detector
array in accordance with the subject invention;
FIG. 4 is a schematic representation of circuitry comprising one
exemplary embodiment of a measuring mechanism according to this
invention;
FIGS. 5a, 5b, and 5c are respective flow chart representations of
the charging and measuring steps according to one exemplary
embodiment of the method according to the present invention;
FIGS. 6a, 6b, and 6c are respective exemplary graphical depictions
of the number of events at various energies occurring within arrays
of exemplary sensitive volumes of respectively decreasing sizes;
and
FIGS. 7a and 7b are schematic representations of an exemplary
integrated circuit implementation of the exemplary circuitry as
represented in FIG. 3 .
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred
embodiments of the invention, examples of which are illustrated in
the accompanying drawings. The following disclosure is for purposes
of example only, and is not intended to limit broader aspects of
the invention embodied thereby.
The apparatus and method of the present invention pertain generally
to radiation monitoring systems, devices, and the like. Although
the following description and appended figures generally refer to
the invention in terms of a personal radiation microdosimetry
device, such reference is not meant as a limitation upon the
invention. For example, the method and apparatus of this invention
are just as relevant to radiation monitoring systems and devices
utilized as area monitors, such as area monitors in radiation work
areas, residences, or automobiles. The present invention also
pertains to monitors for space platforms (i.e., space vehicles
supporting living beings and/or equipment). Furthermore, measuring
and predicting the effects of low level radiation in any
environment are within the spirit and scope of the present
invention. It should be understood that the invention is generally
referred to as a microdosimetry device for ease of illustration
only and that such description is not a limitation upon the
invention. The apparatus and method of this invention pertain to
any use in which a qualitative analysis of a radiation field is
desired.
A microdosimetry device for qualitatively analyzing radiation in a
complex radiation field incident thereon is provided. As depicted
in FIG. 1, microdosimetry device 10 of the present invention may
comprise a personal microdosimetry device 12 compact and portable
enough for an operator to wear, for instance attached to his or her
person as a suitably packaged film badge.
Referring now to FIG. 2, microdosimetry device 12 comprises an
array 14 of microstructure non-volatile memory devices. As embodied
herein and shown for example in FIG. 2, detector array 14 is
packaged in this embodiment as a semiconductor device 16. In the
presently preferred embodiment, semiconductor device 16 comprises a
commercially available erasable programmable read-only memory. It
will be understood by those of ordinary skill in the art, however,
that any equivalent package of microstructure non-volatile memory
devices may be used without departing from the scope and spirit of
the present invention. Thus, it is understood to be within the
scope of the present invention to include any such packaging, such
as, for example, a non-volatile random access memory device or an
ultraviolet programmable read-only memory. For ease of explanation,
the following discussion will refer to an erasable programmable
read-only memory. However, this is not meant as a limitation upon
the present invention.
FIG. 3 is a cross sectional depiction of a floating-gate
avalanche-injection metal-oxide semiconductor (FAMOS) transistor
having a microstructure non-volatile device 18 established upon a
substrate generally 20. Such a transistor may comprise, for
example, one element within array 14 embodied within semiconductor
device 16 as in FIG. 2. As will be understood by those of ordinary
skill in the art, a FAMOS transistor is but one example of a
microstructure non-volatile memory device. It will be understood to
be within the scope and spirit of the present invention to utilize
any equivalent microstructure non-volatile memory devices to
comprise array 14.
Each microstructure non-volatile memory device 18 within array 14
as in FIG. 2 further defines an estimated sensitive volume shown as
shaded area 22 in FIG. 3, within which charge is generated
responsive to incident radiation as is explained in greater detail
below. Although precise dimensions of sensitive volume 22 are at
present unknown, FIG. 3 illustrates a current diagrammatic
estimate.
It should be additionally understood that semiconductor 16 may be a
multipurpose device. For example, a commercially available EPROM
may contain an array 14 of microstructure non-volatile memory
devices 18 which may be employed in any variety of data storage and
retrieval functions. Because each element 18 of such an array 14
within such a device 16 is individually addressable, semiconductor
device 16 may serve multiple functions. Thus, in another preferred
embodiment of this invention, a plurality of microstructure
non-volatile devices 18 may be utilized in analyzing an incident
complex radiation field. The selected or determined area or portion
of array 14 comprising this plurality of microstructure
non-volatile memory devices 18 so employed, whether comprising the
whole or part of array 14, will be hereafter referred to as the
measured area.
Referring again to FIG. 3, each device 18 defines a corresponding
sensitive volume 22. In this embodiment, sensitive volume 22 is
believed to be located between floating-gate 24 and drain 26.
However, those of ordinary skill in the art should understand that
such is presently an estimate. Sensitive volume 22 may be larger
and, for example, extend throughout the oxide between floating-gate
24 and substrate 20. The sensitive volume may, however, be
generally defined as that region within the device within which
charge generated by the traversing radiation particles is
efficiently collected at floating-gate 24, thereby altering the
charge thereon. Such charges are generated by the radiation
particles as a result of collisions with atoms of the oxide
crystalline structure within the sensitive volume 22. The number of
charges generated is proportional to the energy of the traversing
particles.
One measurement peculiar to individual particle types is the linear
energy transfer (LET), which may generally be defined as the energy
deposited per unit pathlength. Thus, regarding microstructure
non-volatile memory device 18 as in FIG. 3, the charge collected at
floating gate 24 is a function of the LET and sensitive volume
size. Specifically, if the device is linear:
wherein
If information concerning the propensity of incident radiation to
damage biological cells is desired, the total energy deposited, or
dose, is an inadequate measurement. Specifically, the amount of
damage depends upon the density of charge ionizations along a
particle trajectory, since that effects the probability of damage
to any given area. Dose equivalent is a unit of radiation that
reflects the harmfulness of radiation. It is the dose multiplied by
an International Commission on Radiological Protection (ICRP)
recommended quality factor that depends on the density of
ionizations along the particle trajectory. Because ionization
densities differ among radiation types, quality factors differ
among radiation types.
Therefore, such qualitative analysis of radiation fields may be
achieved through a spectral analysis providing dose equivalent or
some similar measurement as recommended by the ICRP. Because this
requires association of various radiation types with corresponding
quality factors, discrimination among radiation types is
required.
Regarding a linear device, as indicated by Equation 2, the
relationship between the energy deposited by a traversing particle
and the particle's LET depends upon the size of the sensitive
volume. If the energy deposited by a particle is known and the size
of the sensitive volume is known, the value of the LET may be
determined, thereby identifying the type of radiation. Note,
however, that if the device is not linear, the dimensions of the
sensitive volume depend upon the LET of the incident particle. That
is, the sensitive volume will depend on the energy of the
individual particle. This makes the computation more difficult, but
not impossible.
Sensitive volume sizes of microstructure devices may be determined
using this relationship. For example, a microstructure device may
be bombarded with a radiation having a known LET. By measuring the
collected charge, the sensitive volume size may be determined.
Therefore, while at present the size of the sensitive volume
defined by device 18 is unknown, it may be experimentally
determined by such or a similar approach. However, as discussed
below, it is practically unnecessary to further define the
sensitive volume of device 18 because it is known to be within a
range appropriate to adequately perform a spectral analysis of
radiation incident to biological cell nuclei or DNA genomes. In
this presently preferred embodiment, the respective sensitive
volumes 22 of devices 18 (FIG. 3) are generally not more than one
cubic micrometer each. The present best estimate is that such a
sensitive volume 22 defines a volume of 0.1 micrometer by 1.0
micrometer long by 100 angstroms thick.
To more generally illustrate the idea of a sensitive volume and its
operation within the present invention, an ideal example is
provided. Referring now to FIG. 6a, an ideal pulse height spectrum
for a relatively large microstructure sensitive volume is provided.
It should be noted, however, that the values and dimensions
provided on the plot of FIG. 6a are by way of example only and do
not purport to represent an actual expected ideal pulse height
spectrum. In this ideal relatively large sensitive volume, all
incident particles of a particular type deposit approximately the
same amount of energy in the sensitive volume because each particle
comes to rest within the sensitive volume, thereby depositing all
of its energy.
Referring now to FIG. 6b, a pulse height spectrum corresponding to
a somewhat smaller microstructure sensitive volume is provided. As
the sensitive volume becomes smaller, the incident particles will
collide with fewer atoms in the crystalline structure, and
consequently deposit less energy. In this case, fewer particles
come to rest within the sensitive volume. Therefore, the majority
of particles will pass through the sensitive volume, depositing
less energy than the particles depicted in FIG. 6a. Consequently,
the pulse height spectrum peaks shift to lower energies, and the
spikes flatten out.
As, therefore, the sensitive volume becomes smaller, the average
expected energy deposited becomes less than the expected energy
derived from Equation 2. That is, the peaks begin to move to lower
energies while the spikes spread into broader curves. This
phenomenon is detailed in the Sondhaus and Bond article referenced
above. This result is particularly acute when the sensitive volume
becomes very thin with respect to the incident radiations angle of
incidence.
As the sensitive volume size further decreases, the energy curves
corresponding to particular radiation types spread into broader and
broader curves, resulting in energy ranges within which particles
of the various radiation types may be expected to deposit energy.
Moreover, when particle types deposit similar energies, their
respective energy ranges will overlap.
Referring to FIG. 6c, at sensitive volume sizes approaching the
sizes of biological cell nuclei and DNA genomes, as explained by
Sondhous and Bond, the curves have flattened out, resulting in a
distribution of individual events representing the microdosimetry
regime. Particular particle types still, however, deposit energy
within corresponding energy ranges. Furthermore, the degree of
overlap among energy ranges corresponding to particle types
depositing similar energies increases so that the propensity of
such particles to damage cells approximately equal in size to such
sensitive volumes is approximately equal. At such microstructure
sensitive volume dimensions, therefore, it becomes necessary only
to distinguish among radiation types having significantly different
energies. Consequently, the ICRP, as noted in the above-referenced
ICRP recommendations and Tables 1 and 2, has recommended quality
factors and, similarly, radiation weighting factors, corresponding
to LET ranges and energy ranges for various radiation types.
______________________________________ LET in Water (keV
.mu.m.sup.-1) QF(LET) ______________________________________ <10
1 10-100 0.32(LET) - 2.2 >100 ##STR1##
______________________________________
TABLE 2 ______________________________________ Radiation Radiation
Type Energy Range Weighting Factor
______________________________________ Photons all energies 1
Electrons & muons* all energies 1 Neutrons <10 keV 5 10
keV-100 keV 10 >100 keV-2 MeV 20 >2 MeV-20 MeV 10 >20 MeV
5 >2MeV 5 Protons** 20 Alpha particles, Fission fragmants, Heavy
nuclei ______________________________________ *excluding Auger
electrons emitted from nuclei bound to DNA **other than recoil
protons
Because, as discussed above, the LET is defined as the energy
deposited by a particle per unit pathlength, the size of the
sensitive volume should generally be known in order to determine
the LET of unknown incident radiation. That is, if the size of the
sensitive volume is known, a particle may be identified, through
its LET, by dividing the energy deposited by that particle by the
pathlength through the sensitive volume through which the particle
traveled. However, as noted in appended Table 1, the ICRP
recommended quality factors apply to ranges of LET.
Correspondingly, only an approximate definition of the sensitive
volume size is required. A similar analysis holds also for
conversion by radiation weighting factors as described in appended
Table 2.
Referring again to FIG. 2, a passive microdosimetry detector device
12 for recording the energy deposition of radiation incident
thereto and enabling spectral analysis of a complex radiation field
is depicted. The detector device 12 comprises an array 14 of
microstructure non-volatile memory devices 18 (as in FIG. 3), each
of a plurality of such microstructure non-volatile memory devices
within array 14 defining a corresponding microstructure sensitive
volume 22 (FIG. 3) within which charge is generated responsive to
incident radiation. In a presently preferred embodiment of the
invention, array 14 is packaged as a semiconductor device 16.
Furthermore, semiconductor device 16 comprises an erasable
programmable read-only memory (EPROM). Those of ordinary skill in
the art will recognize that there are many commercially available
EPROMS, for example, ultraviolet programmable read-only devices and
electrically erasable programmable read-only memory devices.
Additionally, semiconductor device 16 may comprise a non-volatile
random access memory or equivalent device.
The choice of any particular device incorporating microstructure
non-volatile memory devices may effect the choice of measurement
methods and devices as described below. However, all such
equivalent alternatives are understood to be within the scope of
the present invention. Therefore, the utilization of an EPROM in
this presently preferred embodiment does not serve as a limitation
thereof.
In operation, EPROM 16 is configured, prior to exposure to a
complex radiation field, such that the exposure will measurably
alter the stored charge level of the microstructure non-volatile
memory devices 18 (FIG. 3) of array 14. For example, referring now
to FIG. 3, each device 18 has an inherent associated threshold
voltage that must exist between the control gate and the drain
before device 18 will be in an "on" state. When device 18 is on,
conduction occurs between drain 26 and source 28. That is, above
such threshold charge, for example, device 18 is on; below such
threshold charge, for example, device 18 is off. Generally,
therefore, each device 18 in array 14 may be initially charged to a
predetermined level so that all devices 18 will be in one state. In
this embodiment, this initial charge is stored on floating gate 24,
an electrically isolated gate within device 18. As devices 18 are
exposed to the complex radiation field, electron/hole pairs (or
charges) are generated within sensitive volume 22, neutralizing
some of the charge on floating gate 24, and bringing the charge on
floating gate 24 closer to the threshold charge. Thus, the change
from the predetermined initial charge to the post-irradiation
charge corresponds to the number of charges generated, and,
therefore, to the energy deposited within device 18 by incident
radiation particles during exposure.
In this embodiment, the floating gates 24 of devices 18 are
initially charged negatively, causing devices 18 to be initially in
an "off" state. It is understood, however, that similar devices may
be configured in such a way as to be initially on or to have
positively charged floating gates. In such configurations, certain
aspects of the following discussion would be altered accordingly as
will be understood by those of ordinary skill in the art. Regarding
this embodiment, then, the negative charge is set by applying a
voltage between source 28 and drain 26 such that electrons from
drain 26 are swept up to floating gate 24 by the more positively
charged control gate 30, which is held at a constant voltage.
Depending on the structure of device 18, the negative charge level
that can be held on floating gate 24 is finite. That is, the
charging process is self-limiting. By charging floating gate 24 to
this self-limited charge level, all devices 18 within array 14 are
easily brought to the same initial charge. It is understood,
however, that such charge level choice is arbitrary.
Upon irradiation, electron/hole pairs are generated in sensitive
volume 22. An electric field across the oxide comprising sensitive
volume 22 sweeps the holes up to floating gate 24, where they
neutralize the negative charges stored thereon. In this manner, the
negative charge on device 18 is reduced towards the threshold
voltage.
In another embodiment of the invention, a measurement device 32 is
operatively associated with detector array 16 as depicted in FIG.
4. Measurement device 32 comprises a communicating device for
communicating with such at least one detector array 14, and a
measurement mechanism for measuring the charge deposition on
microstructure non-volatile device 18. A qualitative analyzing
device, here embodied by personal computer 33, is operatively
associated with such measurement device for converting the charge
deposition to a spectral analysis of the complex radiation
field.
Referring again to FIG. 4, a schematic diagram of one exemplary
embodiment of the above-mentioned measurement device 32 is
provided. The diagram of FIG. 4 is a functional diagram
corresponding to the schematics presented in FIGS. 7a and 7b. In
particular, referring to FIG. 7a, chips U0 and U1 correspond to
interface 34 of FIG. 4; chips U2, U3, and U5 correspond to address
decoder 36; and chip U4 corresponds to buffer 38. Referring now to
FIG. 7b, the transistor devices enclosed within the dashed lines at
40 and 42 comprise relays 44 and 46 of FIG. 4. U1 corresponds to
the detector 16 being measured. In this presently preferred
embodiment, detector 16 may be plugged into a communication device,
shown generally under detector device 16 at 35, electrically
connected to the circuitry of measurement device 32 as depicted in
FIG. 4.
In this presently preferred embodiment, the qualitative analyzing
device operably associated with measurement device 32 is comprised
of a personal computer 33 in communication with measurement device
32 through PC connector 48.
In operation, measurement device 32 and detector device 16 are
shown in a measurement configuration in that device 16 is exposed
to ultraviolet light from ultraviolet lamp 50. More particularly,
address decoder 36 receives instructions from the external PC 33
through PC connector 48, whereupon address decoder 36 translates
such instructions into a form acceptable by and actable upon device
16 and/or relays 44 and 46. These instructions are then routed to
detector 16 by control lines 52, address lines 54, and relay
control lines 56, as will be understood by those of ordinary skill
in the art. Data lines 58 are utilized to retrieve measurement
information from device 16 through interface 34 to data buffer 38,
from which the stored information will be read by the external PC
33 through PC connector 48 according to the PC's operating
parameters.
It should be understood that the configuration of the circuitry of
measurement device 32 as depicted in FIG. 4 is not a limitation
upon the present invention. Such configuration may vary, for
example, with the use of varying semiconductor detector devices 16
as described above. For example, those of ordinary skill in the art
will recognize that control lines 52 may additionally comprise a
program enable line if device 16 comprises an EPROM. Furthermore,
the program voltage supplied through relay 44 over a program
voltage line 58 may be required by device 16 to perform various
functions as is understood in the art. Relays 44 and 46,
consequently, provide possible voltage alternatives as necessary
and are controlled by the external PC 33 through address decoder 36
and interface 34 over relay control lines 56.
Additionally, the circuit of measurement device 32 as in FIG. 4
serves as a charging means for charging the plurality of
microstructure non-volatile memory devices 18 (FIG. 3) to a
predetermined initial charge prior to exposure to a complex
radiation field. Specifically, the charging process is directed
from the external PC 33 by instructions directed through PC
connector 48, address decoder 36, and interface 34 over control
lines 52 and address lines 54 and utilizing the program voltage
from program voltage line 58. It will be again understood, however,
that there are various ways of setting such predetermined initial
charge to the individual microstructure non-volatile memory devices
18 comprising array 14 within device 16, depending on the nature of
device 16 and microstructure non-volatile memory devices 18.
As is apparent from the discussion above, therefore, the circuitry
of measurement device 32 as in FIG. 4 may serve both as a charging
means and as a measurement mechanism with respect to a detector 16
through the communication device 35 of measurement device 32.
It is also considered to be within the scope of the present
invention that semiconductor device 16 or individuals of
microstructure non-volatile memory devices 18 may be shielded with
varying types and degrees of shielding material, such as tissue
equivalent plastic, so that the charge collected within
microstructure non-volatile memory devices 18 simulates that
collected within specific biological tissue. Such shielding
material may also be used to simulate the environment within which
it is desired to qualitatively analyze a radiation field. One
exemplary embodiment of shielding material surrounding a sensitive
volume within a microdosimetry device is presented in FIG. 5 of the
above-referenced U.S. Pat. No. 5,256,879. Similar shielding
material, as indicated in phantom at 59 in FIG. 3, may be utilized
to shield the array 14 or a portion thereof. Shielding 59 is
represented disagrammatically and is not intended to represent
particular shielding dimensions or materials.
The shielding material may also be utilized to customize array 14
to represent any number of operating environments or to simulate
any type of biological tissue. For example, neutrons could cause
significant damage if inhaled into the lung or swallowed into the
stomach. Thus, to simulate an internal organ being subjected to an
incident neutron, microstructure non-volatile memory device 18
would be heavily shielded with tissue equivalent shielding
material. Likewise, to simulate a cornea of an eye being subjected
to an incident neutron, microstructure non-volatile memory device
18 would be relatively unshielded.
In further accordance with the present invention, a method for
qualitatively analyzing a complex incident radiation field is
provided. The method comprises the first step of subjecting to the
radiation field a detector array comprised of a plurality of
microstructure non-volatile memory devices each having a
corresponding plurality of associated microstructure sensitive
volumes. Furthermore included is the step of measuring the charge
deposition on at least one of the microstructure non-volatile
memory devices, the charge deposition being generated responsive to
the incident radiation within the microstructure sensitive volumes.
The measuring step provides data corresponding to the charge
deposition on each individual measured microstructure non-volatile
memory device. Such data is then output for converting the charge
deposition to a qualitative analysis of the complex radiation
field.
In a preferred embodiment of the method of this invention, the
method further comprises converting the charge deposition into an
estimate of the number of events within discrete energy bands
occurring within the measured area of the array of microstructure
non-volatile memory devices and translating the estimate into a
spectral analysis of the complex radiation field. An event is
defined as the traversal by a radiation particle of a sensitive
volume 22 as in FIG. 3. An energy band is an energy range within
which particles of certain radiation types may be expected to
deposit energies as they traverse sensitive volumes having
dimensions within a certain range, as described above. The spectral
analysis may be achieved by applying quality factors or similar
measures of the propensity of incident radiation to cause damage to
applicable volumes of a size similar to the sensitive volumes, to
the associated energy bands, as described above.
The method additionally comprises the step of charging to a
predetermined initial charge, prior to the subjecting step, the
microstructure non-volatile memory devices whereby the
predetermined charge is altered during the subjecting step
responsive to incident radiation. The predetermined initial charge,
as described above, is set beyond a known threshold charge. The
threshold charge is a charge level inherent to the microstructure
non-volatile memory device 18, the traversal of which causes device
18 to change state. Therefore, as described above, radiation
incident to a microstructure non-volatile memory device 18
initially holding the predetermined initial charge causes such
charge to be altered towards the threshold charge such that a
correlation exists between the post-irradiation charge held by
microstructure non-volatile memory device 18 and the energy
deposited in such device by incident radiation.
Accordingly, the method of this presently preferred embodiment
further comprises the steps of changing the post-irradiation charge
on the at least one microstructure non-volatile memory device 18 at
a known rate toward the threshold level and determining the time
required for such device to change state. As described below, there
are various methods for altering the post-irradiation charge,
depending, for example, on the nature of device 16 (for example,
FIG. 2) and microstructure non-volatile memory device 18 (for
example, FIG. 3).
Referring now to FIGS. 5a, 5b, and 5c, one exemplary embodiment of
the method according to the present invention is provided. FIGS.
5a, 5b, and 5c depict, primarily, a flow chart functionally
describing a computer program performable by, for example, the
external personal computer 33 described above in association with
FIG. 4. Furthermore, the steps of the method depicted by this
embodiment are enacted through, for example, the circuitry of the
measuring mechanism and charging means of measurement device 32 as
in FIG. 4.
In operation, program variables are initialized at 60 and the main
program loop is entered at 62. If at 64, the charge function is
indicated, the charge routine as depicted in FIG. 5b is entered at
66. Referring now to FIG. 5b in conjunction with FIG. 4, interface
34 is initialized at 68; array address variables are initialized at
70; and detector 16 is addressed at 72 through address decoder 36,
interface 34 and address lines 54. In this presently preferred
embodiment, the address variables correspond to blocks of eight
microstructure non-volatile memory devices 18 within detector 16.
This format is, however, as will be understood in the art, merely a
function of the nature of device 16 and may be varied in accordance
therewith.
Once the first block of devices 18 on detector 16 has been
addressed, write signals are activated at 74 over control lines 52.
This instruction configures the block of memory devices 18 to be
charged to the predetermined initial charge. The physical enactment
of the charging process is a function of the nature of device 16
and memory devices 18 and may be accomplished through specific
signals on control lines 52 and, for example, program voltage line
58.
As described above, the charging process is self-limiting.
Accordingly, the program delays at 76 to permit the predetermined
initial charge to accumulate. The write signals are deactivated at
78 and the next block of eight memory devices 18 are addressed at
80. Because the device 16 of the presently preferred embodiment
comprises 65,536 microstructure non-volatile memory devices 18,
there are 8192 blocks of eight such devices 18 addressed zero
through 8191. Therefore, when the address variable reaches 8192 at
82, the program exits at 84. Otherwise, the program returns to 74
and continues the loop.
Returning now to FIG. 5a, following the exit of the charge routine
at 66, detector device 16 may be exposed to a radiation field at
86.
Referring additionally to the associated electronics depicted in
FIG. 4, after detector device 16 has been exposed to the radiation
field and placed for measurement in communication with measurement
device 32, a read instruction will be given at 64, causing the
program to enter at 88 the read routine depicted by FIG. 5c.
As discussed above, the readout process embodied by the method
according to this presently preferred embodiment of the invention
is a destructive procedure. That is, the post-irradiation charge
held by microstructure non-volatile memory devices 18 is reduced at
a known rate toward a threshold charge, the traversal of which
causes the devices 18 to change state. Thus, the time required to
cause a device 18 to change state during the readout process after
exposure to an incident radiation field corresponds to the energy
deposited in the device by the incident radiation field.
Accordingly, a timing variable is initialized as the detector
charge reduction begins at 90. In this embodiment, the charge is
reduced on all micro-structure non-volatile memory devices 18
within detector array 16 simultaneously by the application of
ultraviolet light by ultraviolet light source 50. It will be
understood by those of ordinary skill in the art that upon exposure
to ultraviolet light, the microstructure non-volatile memory
devices 18 (FIG. 3) comprising array 14 (FIG. 2) of detector device
16 lose negative charge on floating gate 24 at a known rate.
It will, furthermore, be understood by those of ordinary skill in
the art that various methods of altering the charge on memory
devices 18 may be employed. For example, it will be understood
that, depending upon the nature of detector device 16 and memory
devices 18, measurement device 32 may be configured to alter the
charge on memory devices 18 through a current tunnelling technique.
The current tunnelling technique has the additional advantages of
creating a more accurate discharge rate and avoiding the
inconvenience and possible health hazards associated with
ultraviolet light source 50. Furthermore, a controlled charge
alteration rate may also be achieved by imposing an electric field
across sensitive volume 22 (FIG. 3) in opposition to the electric
field holding the stored charge on floating gate 24. Accordingly,
such charge altering method are indicated as alternatives to block
86 at 86' and 86". Additionally, as discussed above, devices 18 may
be of a type having positively charged floating gates, thus
requiring appropriate procedural alterations as will be understood
by those of ordinary skill in the art. All of these and other
equivalent methods of altering the charge on microstructure
non-volatile memory devices 18 are understood to be within the
scope and spirit of the present invention.
Following the timing initialization at 90, interface 34 is
initialized at 94 and a pass number is initialized at 96. The pass
number is a record of the number of measurement passes made through
measurement array 14 within detector device 16. In general, the
routine depicted by FIG. 5c repeatedly checks blocks of
microstructure non-volatile memory devices 18 having changed
states. By recording the time from timing variable initialization
at 90 to the change of state, or failure, of an individual memory
device 18, as described above, the energy deposited by incident
radiation within that individual memory device 18 may be
determined.
Accordingly, the address variable is initialized at 98. As
described above, the method addresses blocks of eight memory
devices 18. Thus, the first block of 8 memory devices 18 is
addressed through address decoder 36, interface 34, and address
lines 54 at 100. When read signals are activated at 102 through
control lines 52, data representing the present state of the
addressed block of eight memory devices 18 are loaded at 104
through data lines 106 through interface 34 to buffer 38. Such data
is read by the external PC 33 through PC connector 48 and is
compared to data stored by the external PC representing the
expected states of the memory devices 18 within the block.
Initially, in this presently preferred embodiment, the expected
states will all be "off." If no discrepancy, or failure, is
detected at 108, therefore, no memory device within the block has
changed state. Therefore, the address variable is incremented at
110 and a check is made at 112 whether the final block of memory
devices 18 have been measured. If not, the next block is addressed
at 100 and the loop continues. If the last block has been read at
112 the current pass through array 14 has been completed, and,
consequently, the pass number is incremented at 114 and the next
pass is initiated at 98.
If a failure is detected at 108, meaning that at least one memory
device 18 within the block has changed state, the loop variable is
initiated at 116. The loop variable in this preferred embodiment
runs from zero to seven and corresponds to each of the eight memory
devices 18 within the block. Therefore, any individual
microstructure non-volatile memory device 18 within array 14 may be
addressed through the address variable and the loop variable. If
the individual memory device 18 addressed by the address variable
and the loop variable has failed at 118, the array pattern stored
by the external PC is updated at 120. That is, on subsequent passes
it will be expected that this particular memory device 18 will have
failed. If, for example, the memory device 18 corresponding to the
first loop variable within a given block has failed on this
particular pass, and there are at present no other memory devices
18 within the block which have failed, the program on the next pass
will expect to see one failure within the block and, therefore,
more than one failure will be required at 108 to trigger step
116.
Referring again to step 120 after a failure is detected at 118, the
failure count is incremented and the failure time is recorded at
122. The failure time at 122 is recorded by the external PC for the
particular memory device 18 identified by its address variable and
loop variable. Therefore, the program stores the failure time for
each individual memory device 18 within array 14. As explained
above, this failure time corresponds to the amount of energy
deposited in each memory device 18. Thus, the individual energy
deposition in each memory device 18 and the positional charge
distribution throughout array 14 is recorded.
If at 124 the failure count equals 65,536, all memory devices 18
within array 14 have failed and the program exits at 126. If the
failure count is not yet full, the loop variable is incremented at
128. If the loop variable is not yet equal to seven at 130, the
routine returns to 118. If the pass through the block under measure
is completed at 130, the routine returns to 110.
The method according to the above-described preferred embodiment
encompasses a destructive readout of detector device 16. That is,
the post-irradiation charge stored on each of microstructure
non-volatile memory devices 18 is altered as described above to
measure the change in stored charge level responsive to incident
radiation, thereby requiring a subsequent recharge to the
predetermined initial charge before detector 16 can be reused. Such
a configuration is compatible with systems wherein a plurality of
detector devices 16 are used and occasionally measured by remote
measurement devices such as measurement device 32 as in FIG. 4. It
may, however, be desirable to nondestructively read detector
devices 16. For example, a visual or audible alarm might be
desirable to alert the wearer of a personnel microdosimetry device
that a specific dose equivalent level has been attained. Such a
configuration would require associated electronics such as
measurement device 32 and, consequently, an associated power source
to continuously read detector device 16.
Accordingly, another possible embodiment of the method of the
present invention would be utilized with a measurement device
similar to, for example, measurement device 32 as in FIG. 4. As
will be understood by those of ordinary skill in the art from the
general description of the possible embodiment below, such
circuitry as is depicted in FIG. 4, or various equivalent
configurations or devices, may be employed to perform a
non-destructive read of detector device 16. The nature of such
equivalent configurations and devices will depend upon the
circumstances of the intended use. For example, measurement device
32 might be appropriate for non-destructive read applications
wherein device 16 is monitored periodically. The construction of
measurement device 32 may, however, be altered, for example for
power and size constraints, if the measurement device were to
constantly monitor device 16. In a personnel microdosimetry
application employing such an embodiment, a power source and a
measurement device would be packaged together with detector device
16.
Generally, the method according to the present invention to perform
the non-destructive read comprises the steps of addressing a first
non-volatile memory device 18 of detector device 16, reading the
state (either on or off) of the device 18, decrementing the bias on
the control gate of device 18 by a small predetermined value, and
recording the control gate bias at which the device 18 under
measure changes state, thereafter repeating the procedure for each
device 18 within the measured area of detector device 16. As is
explained in more detail below, the control gate bias at which the
device 18 changes state corresponds to the charge stored on the
floating gate 24 (FIG. 3) therein and, therefore, to the energy
deposited by incident radiation.
As with the presently preferred method encompassing the destructive
read described above, an initial detection that a device 18 has
changed state precludes further read procedures with respect to
that device. Thus, regarding this possible embodiment, if the
device 18 under measure has, on the initial read, changed state
from its predetermined initial state, its control gate bias is not
decremented.
The above-described non-destructive read would operate through
altering the electric field between the floating gate and the
substrate. As is known in the art, this electric field determines
the state of the device and may be controlled by the bias on the
control gate without affecting the charge collected on the floating
gate. The intensity of this electric field is, however, determined
by this charge. Thus, the change in the intensity of this electric
field corresponds to the change in charge collected on the floating
gate. Accordingly, the bias decrement required to changed the state
of device 18 also corresponds to the collected charge.
As described above, the method according to one presently preferred
embodiment individually measures groups of memory devices 18 within
array 14. In the presently preferred embodiment as depicted in
FIGS. 5a, 5b and 5c, the entire 65,536 element array is measured.
However, it is to be understood to be within the scope of the
present invention to measure selected portions of detector array
14. Such measured portions may be referred to as the measured area.
Such a procedure may be desired if, for example, detector 14 is
utilized as a personnel radiation detector. If a dangerous
radiation level as indicated by, for example, a dose equivalent
estimate generated from the measurement taken from the measured
area is detected, it may be desirable to preserve the charge
distribution held by the remaining unmeasured portion of array 14
for subsequent confirmation. Additionally, the charge routine as
depicted in FIG. 5b may also address desired portions of array 14
without affecting the charge stored on the remainder of the array.
Thus, it may be desirable to periodically measure and reset a
measured portion of array 14, while leaving the remainder of the
array as a cumulative microdosimeter for a longer period of
time.
Because detector device 16 as shown in FIG. 2 is a passive
microdosimetry device, charges deposited in memory devices 18 (FIG.
3) may be integrated during exposure to a complex radiation field.
That is, it is possible that multiple particles may traverse the
sensitive volume 22 of a single memory device 18, resulting in an
accumulation of charge. It is anticipated, however, that such
occurrences will generally be so infrequent or inconsequential as
to have little effect on the accuracy of a dose equivalent or
similar estimate. If, however, a detector device 16 were to be
utilized in a radiation field wherein such charge integration would
effect the accuracy of such estimates, another preferred embodiment
of the present invention adequately accommodates such integration
in translating to a dose equivalent or similar estimate.
Generally, because the positional charge distribution may be
derived as described above, radiation patterns may be observed.
Thus, for example, a particular area of array 14 may exhibit energy
depositions indicative of single or multiple hits by gamma
radiation. Accounting for and subtracting these energy depositions
from the positional energy distribution corresponding to array 14,
other energy depositions indicative of other particle types, for
example neutrons, may appear. These energy depositions may, in
turn, be indicative of single or multiple hits by such other
particle types. It will be understood that a variety of
probabilistic algorithms may be developed to analyze such
positional charge distributions. It is, therefore, to be understood
that any and all such equivalent techniques are within the scope
and spirit of the present invention.
The ability of the present invention to measure positional charge
distribution may permit its use in applications currently employing
autoradiographic techniques. As is known in the art,
autoradiography involves utilizing photography-like methods in
analyzing radiation incident to, for example, biological tissue. A
slice of such affected tissue may be taken and covered in an
appropriate emulsion. When the emulsion gels, the slice is
"developed." Because the radioactive material affects photographic
film, a "photograph" of the radiation pattern may be obtained.
As discussed above, the present invention may provide an analysis
of the positional charge distribution on an affected array. Thus,
such an array may permit analysis of radiation distribution
patterns without resort to autoradiographic methods. In particular,
specific types of tissue may be simulated using shielding
techniques as discussed above.
Furthermore, it will be apparent to those skilled in the art that
various modifications and variations can be made in the apparatus
and method of the above-described presently preferred embodiments
without departing from the scope or spirit of the invention. For
example, as described above, various realizations of a measurement
device might be required, depending upon the nature of the detector
device containing the array of microstructure non-volatile memory
devices. Thus, it is intended that the present invention cover
modifications and variations of this invention as would be apparent
to those of ordinary skill in the art, as would come within the
scope of the appended claims and their equivalents.
* * * * *